Disentangle beginning-of-buffer

Like other complex-looking expressions, the conditional expression
within beginning-of-buffer can be disentangled by looking at it
as parts of a template, in this case, the template for an if-then-else
expression. In skeletal form, the expression looks like this:

The true-or-false-test of this inner if expression checks the
size of the buffer. The reason for this is that the old version 18
Emacs used numbers that are no bigger than eight million or so and in
the computation that followed, the programmer feared that Emacs might
try to use over-large numbers if the buffer were large. The term
`overflow', mentioned in the comment, means numbers that are over
large. More recent versions of Emacs use larger numbers, but this
code has not been touched, if only because people now look at buffers
that are far, far larger than ever before.